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Linear Motion Mechanisms of Lead Screws
Linear Motion Mechanisms and Lead Screw Design at PBC Linear

Linear Motion Mechanisms and Lead Screw Design at PBC Linear

Leadscrew

Design advancements for linear motion systems

A linear motion system achieves higher performance through consistency and precision along the entirety of its mechanical and electromechanical design elements. This group of elements includes the design and manufacture of the lead screw, anti-backlash nut, couplings, motor, and control strategy. By understanding each of these components and their impacts on design output, as well as the difference between traditional and advanced manufacturing processes, customers will enjoy better success in meeting their application demands.

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Quality of lead screw and nut

Lead Screw technology has been in use for decades, along with a variety of nut designs and materials. Traditional manufacturing methods, which are still predominately in place today, involve manual adjustments that limit the quality of the lead screw to the skill level of the operator.

PBC Linear has embraced the modern manufacturing model by automating this process through metrology, shaft-straightness, and surface finishing. Metrology is a key advancement within the modern lead screw manufacturing model. For transport-type applications where an object is moving point-to-point along an axis, the traditional method of checking lead accuracy every 300 mm (six inches) is adequate.

In contrast, high-precision positioning applications require accuracy within the rotation of a single thread. Any deviation from the suitable thread geometry is considered an erratic pitch error, or commonly known within the industry as the drunkenness of the thread.

Our automated CNC manufacturing equipment and detailed inspection methods produce tighter control and quality so that the high and low point within an individual thread show greatly improved sub-rotation accuracy — in other words, less drunkenness. This in turn helps lead screws hold positioning repeatability over a single rotation to within microns. This is an especially critical performance metric in applications that require sub-rotation precision, and may include:

  • 3D printing, where layer resolutions are being pushed as low as 10 µm per layer.
  • Medical devices such as syringe pumps, where dispensing units must output lifesaving medications and control doses down to microliters.
  • Optical and scanning equipment, semiconductor chip and wafer processing equipment, and lab-automation, where tight accuracies are needed.

After the thread rolling is complete, screw-shaft straightness must be measured and adjusted.

Leadscrew Design Elements Figure 1

Straightness is critical because any error is accentuated when it is assembled with the motor, and can cause vibration, noise, and premature wear. Traditional manual methods of screw straightening can produce a snow-cone effect in the screw-shaft geometry, forming a single arch or multiple arches that corkscrew around the long shaft axis.

In contrast, automated straightening and inspection processes eliminate these errors resulting in stable screw performance. Further high performance is added with the application of PTFE coating. A consistent smooth finish avoids pitting, fissures, bubbles, flaking, or surface roughness that can cause premature wear in the nut.

Interaction of the nut and screw

Traditional anti-backlash nuts use a multi-piece design that requires a coil spring to generate axial force, which is then converted to radial force through mechanical interference. The design relies upon injection molded components to apply force equally to the fingers, controlling the fit between the screw and nut. Problems that contribute to failure in these designs are:

  • sporadic and variable force of the spring
  • dramatic preload changes in the first 1,000 cycles
  • stick-slip of the collet on the nut
  • fluctuating pressure as the nut material wears
Leadscrew Design Elements Figure 2

In contrast, our Constant Force® nut includes a simplified two-piece design that applies pressure to the nut fingers in a radial fashion, which is the direction needed to control clearance or play between the nut and screw. A constant force spring design ensures consistent pre-load over the life of the axis, while the self-lubricating nut material with PTFE offers consistent lubricity and enhanced efficiency.

A big advantage of our constant-force anti-backlash lead screw nut is its ability to be tuned to a specific application using adjustments to the spring and other parameters. This tuning allows for the optimization of preload, backlash, drag force, and running clearance to meet required specifications. Each screw and nut combination, along with each motor and screw assembly, can be tested for each of these performance characteristics during validation and final inspection.

Coupled or direct connection to drive

There are three basic ways a screw attaches to a motor. The traditional method uses a coupler as the component between the screw and a motor built with an extending stud shaft. There are some drawbacks to this method of assembly, including:

  • multiple components
  • greater space requirements
  • potential alignment issues
  • lower performance and system life

Another method uses a tapered bore to mechanically secure the screw in place. These assemblies are common on motors that require frequent maintenance and a quick method for disassembly and reassembly. The drawbacks include:

  • inconsistent alignments
  • snow-cone effects
  • excessive maintenance
  • premature system failure

The advanced method uses a direct fit between the screw and the motor. In this scenario, the screw is inserted into a hollow shaft within the motor and then permanently fixed near the back of the motor using an industrial adhesive. This method ensures:

  • fewer components
  • maximum engagement between screw and motor
  • highest accuracy alignment possible
  • least amount of runout
  • low maintenance and extended life

Selection of motor type and design

Linear actuators come with a choice of motor options, with the most common being open loop steppers, closed loop steppers containing either a board mounted control or smart stepper, or brushless DC (BLDC) motors. Each has its own performance proposition regarding speeds and load capabilities, and each comes with their own set of pros and cons around cost, integration, and control.

Typical general-purpose motors use a wavy washer to hold bearings and the assembly in place. While they are usually adequate for rotary applications, wavy washers can cause small amounts of axial or linear play, resulting in inaccuracies of linear position. To alleviate this, one or both of two elements can be modified in the design. Larger bearings can be inserted to increase the thrust load capability of the assembly, and a spanner nut can be added and adjusted to a predetermined torque specification to take the play out of the system.

Leadscrew Design Elements Figure 4

Choice of control options

The last advancement in the modern manufacturing model is how the physical linear motion mechanism is directed and controlled. Traditionally, these need multiple separate pieces including an amplifier and controller. Each needs a cabinet and the associated hardware, wiring, encoder, and sensors for feedback, making them complicated and cumbersome to install, troubleshoot, and operate.

Off-the-shelf smart motor solutions have simplified the wiring and have reduced the number of connectors and sensors associated with gaining step-servo type performance and control. This provides cost savings, thanks to a lower component count and less installation time and labor. These motors also come in preassembled industrialized packages that seal and protect the board and control from abuse or contamination. For applications requiring specific parameters, a custom unencapsulated IP20 motor-mounted board control is a useful option. This is especially true for large-volume applications placed in stylized housings or equipment. These actuators utilize the advantages of smart motors, where control is right at the motor for more efficient communication with the master or PLC.

More information

To find more in-depth information and application examples on this topic of lead screw assemblies, please read the Design World article: Linear motion systems: Only as strong as weakest link.